1. Field of the Invention
The present invention relates generally to composite gels and aerogels and more specifically to mesoporous composite gels and aerogels and their various uses.
2. Description of the Background Art
Xerogels and aerogels derived from the condensation and hydrolysis of metal alkoxide precursors have been studied for a variety of applications, including uses as optical, thermal, and electronic materials. Aerogels, because they are highly porous (80-99% by volume) and have a high surface area (up to 1000 m2/g), are especially well-suited to catalytic and sensing applications, where rapid transport of reactants (or detectable species) and large, accessible surface areas are critical to performance. In composite xerogels and aerogels, the gel structure can act as a host material for immobilized guest particles that perform catalytic, electrochemical or chemical sensing functions.
Typically, guest materials such as catalytic particles have been incorporated into xerogels and aerogels either by adding the guest material or a guest material precursor to a sol-gel precursor mixture before a sol-gel is formed or by impregnating materials into an already-formed xerogel or aerogel. A disadvantage to the method of adding a guest material to a sol-gel precursor mixture prior to forming a sol-gel is that the components may become so thoroughly mixed that the particles of the guest material become completely encapsulated by the sol-gel precursor material. Such encapsulation reduces the exposure of the particles of the guest material to the inner surface area of the subsequently formed gel and thus reduces the effectiveness of the composite for its intended use as a catalyst, sensor, fuel cell, etc. Further, thorough and prolonged mixing of a particulate guest material with a sol can lead to the loss of critical properties, particularly transport properties (which require intimate contact between guest particles) and chemical properties (which involve guest interaction with molecules in the mesopores). A disadvantage to the method of impregnating materials into an already formed aerogel is that the incorporated guest material may leach or wash out of the aerogel.
Accordingly, it is an object of this invention to provide new composite materials in which a guest solid particulate is fixed within a porous matrix.
It is an other object of the present invention to provide a new composite material in which a guest material fixed within a porous matrix can interact with an infiltrate within the matrix.
It is an other object of the present invention to provide a new composite material in which a guest material is incorporated into a porous matrix so that leaching or washing out of the guest material is minimized.
It is a further object of the present invention to provide new composite materials for use as catalysts; porous black composites (e.g., for blocking stray light); power source electrodes and electrode structures (where the term power source includes batteries, fuel cells, electrolytic capacitors, supercapacitors, photovoltaics, thermophotovoltaics, hybrid battery capacitors, etc.); thermoelectric materials; and chemical, optical, physical and biological sensors.
It is an additional object of the present invention to provide new, nanoscale porous composite materials that achieve transport paths for conductivity of ions, molecules, electrons, phonons, combinations thereof, etc., from guest-to-guest through the microstructure of the aerogel at low volume percentages of particulate guest.
These and additional objects of the invention are accomplished by commingling a particulate guest (such as a colloidal or dispersed (i.e., non-colloidal) solid or a powder) with a sol which is either about to gel or in which gelation has just started. After addition of the particulate, the mixture is then permitted to gel into a solid, gelled composite with open pores. This solid, gelled composite is then dried in a manner that prevents the collapse of open pores within the solid, gelled composite in which the gel acts as a xe2x80x9cnanogluexe2x80x9d that holds the particles together. Introducing the guest particulate into a sol and forming a gel in this manner prevents encapsulation of the guest particles by the sol material while sufficiently incorporating the guest particles into the gel network so that the guest material does not leach or elute out during subsequent processing steps or during the subsequent use of the composite. The bulk and surface properties of both the guest material and sol material are retained on the nanoscale . The transport- and density-dependent properties of the composite gel can be tuned by varying the volume fraction of the guest material, thereby increasing the design flexibility of these nanoscale materials for optical, chemical, thermal, magnetic, and electronic applications. The chemical and physical properties of the composite material can be further engineered at multiple points during sol-gel processing by modifying the host solid, the guest solid, the composite gel, or the composite aerogel.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Throughout this application, all references cited are incorporated by reference in their entirety and for all purposes.
Typical precursors for gels or aerogels are metal alkoxides represented by the general formula (M(OR)n). For silica structures, the typical precursor is an oxysilane represented by the general formula (Si(L)4-n(OR)n, where R is organic (typically alkyl), where each xe2x80x94OR may be the same or different if more than one xe2x80x94OR is attached to the silicon, where n is an integer having a value of 1 to 4, and where L is any group other than xe2x80x94OR.
As used herein, the terms xe2x80x9csolxe2x80x9d, xe2x80x9cgelxe2x80x9d, xe2x80x9cxerogelxe2x80x9d and xe2x80x9caerogelxe2x80x9d are used in their commonly accepted meanings. In particular, the term xe2x80x9csolxe2x80x9d refers to a colloidal suspension of precursor particles and xe2x80x9cgelxe2x80x9d refers to a wet three-dimensional porous network obtained by condensation of the precursor particles. Examples of sols include, but are not limited to silica sols, zirconia sols, vanadia sols, manganese oxide sols, magnesia sols, niobium oxide sols, alumina sols, tungsten oxide sols, yttria sols, tin oxide sols, cobalt oxide sols, nickel oxide sols, ceria sols, titania sols, calcia sols, aluminosilicate sols, or mixtures thereof. The sol could also be an non-oxidic or organic sol. As used herein, the term xe2x80x9cnetworkxe2x80x9d is defined conventionally to mean a solid frame that sustains its shape and weight in the environment in which it is formed. That environment is the volume defined by the liquid phase precursors (solvent and any solutes) filling the vessel used for gelation. In the specification and the claims that follow, the onset of gelation is defined as the time at which the colloidal particles that comprise the sol (not to be confused with colloidal guest particles) begin to link together in the reaction volume. This point is accompanied by an increase in viscosity. In xerogels, the gel is dried under ambient conditions, leading to collapse of the pores, densification of the oxide structure and considerable shrinkage. In aerogels, the gel is dried under supercritical conditions to form a high surface area, high-porosity, ultra-low-density material. In supercritical drying, the pore-filling liquid is taken above its supercritical temperature and pressure before extraction, which prevents capillary forces from developing and then collapsing the pores of the gel. Other methods for preventing the collapsing of the pores and for forming aerogels are known, including evaporation of low surface tension liquids from the pores, freeze-dry extraction of the pore fluid, the addition of a low surface tension agent followed by evaporation, silanization of the wet gel followed by evaporation, etc.
In the present invention, a gel composite is formed by adding a guest particulate to a sol at or near the onset of gelation. The guest particulate may be in the form of a dispersed particulate, colloidal suspension or powder.
As used herein, the term xe2x80x9cdispersed particulatexe2x80x9d refers to a non-colloidal particulate in which material is retained in a liquid phase without substantively complete visible settling, i.e., in which settling has not reached equilibrium. The degree of settling that is permissible depends upon the intended use for the composite aerogel. Where good transport properties from guest-to-guest are critical, visible settling should be minimal or negligible. If transport properties are not critical, the extent of settling may be greater. In some instances, it may be useful to allow settling of the particulate guest until a gradient of the particulate within the liquid-solid suspension develops.
As used herein, the term xe2x80x9ccolloidal suspensionxe2x80x9d refers to a suspension of particulates that does not undergo settling under the conditions existing at the onset of gelation. Throughout the specification and claims, the network of an aerogel refers to the frame (i.e., the solid portion) of the aerogel that defines the pores. The frame of the aerogel does not include the pores or material trapped in the pores. A particulate is said to be incorporated into the network of an aerogel if particles of the particulate form part of the frame of the aerogel. This definition excludes conventional structures in which particulates are added after substantial gelation (and, consequently, matrix formation) has occurred. In those prior art structures, by the time the particulates are added, the network has already developed to the extent that the added particulates may, at best, form a deposit on or coat the matrix frame and do not form a part of the matrix frame.
The timing of the commingling of the sol and particulate guest should be such that the dispersed particulate or, colloidal particles may be incorporated in the growing network. If commingling with the particulates occurs too much before gelation, the particles may agglomerate and/or settle to the extent that they can no longer be incorporated in the network of the aerogel once gelation begins; further, if the particulates are commingled with the sol (or its molecular precursors) too soon before gelation, the particulates risk a high level of coverage or encapsulation by the sol. If commingling with the particulates occurs too late after the onset of gelation, the network of the aerogel will be too well-formed to incorporate the particulates.
Typically, the desired incorporation of the particulates (dispersed particles, colloidal particles, or powders) occurs if the particulate phase is commingled with the sol within one-half hour of gelation (particularly if the particulate guest is added to the sol). More often, the desired incorporation of the particulates (dispersed particles, colloidal particles, or powders) occurs if the particulate phase is commingled with the sol within 10 minutes of gelation. Most often, the desired incorporation of the particulates (dispersed particles, colloidal particles, or powders) occurs if the particulate phase is commingled with the sol within three minutes of gelation. If the particulate is large enough to settle, then it may be advantageous to lightly agitate (by shaking, stirring, etc.) the sol/guest mixture immediately after the mingling of the particulate with the sol. The duration and degree of agitation depends on the intended end use of the composite. Where a homogenous composite without transport paths is desired, heavier agitation for a longer duration assists in providing the desired homogeneity. If desired, agitation may be continued throughout the gelation process. If transport paths are desired, it may be best to only lightly agitate the sol/guest mixture, and the agitation is best completed before complete gelation occurs.
Any size of particle may be incorporated into the network of the present composite. Typically, average particles sizes incorporated into the network of the present composite are up to about 1 mm, and are more often about 1 nm up to about 100 xcexcm. The volume percent of the particulate guest that is added to the sol may be above or below a threshold for electrical, thermal, or ionic conductivity.
The particulate may be any powder, dispersed particulate, or colloidal suspension, regardless of chemical composition, although the particulate is preferably insoluble in and nonreactive with the solvent for the sol. Some typical particulates include Pt; Au; TiO2; SiO2; Ag; Cu; Al; Fe; RuO2; Si; GaAs; ZnO; CdS; C (any carbon allotrope, such as graphite, diamond, fullerenes, nanotubes, blacks, soots, vitreous carbon, coke); Pd; Bi2Te3; high molecular weight polymer, including, but not limited to polymethyl methacrylate; zeolites, including, but not limited to a synthetic type Y faujasitic aluminosilicate zeolite; mesoporous ceramics other than mesoporous ZnO, mesoporous TiO2, or mesoporous RuO2; and mixtures (homogeneous or segregated) thereof. The particles of the particulate may be composites, and also, more than one species of particulate may be commingled with the sol.
In one embodiment of the invention, a particulate (dispersed particulate, colloid suspension, or powder) is poured or otherwise added to a sol shortly before or shortly after the onset of gelation. This method works well when the particle sizes of the particulates are less than about 1 xcexcm. Particularly for larger particle sizes, it may be best to add the sol to the particulates (which may or may not be dispersed and which may or may not be suspended in a liquid-phase; for example, the particulates may be in the form of a bed of powder) and then commencing gelation of the sol, preferably simultaneously with addition or shortly thereafter. When the sol is added to a particulate volume, the amount of time between addition and the onset of gelation is less critical than when the particulate is added to the sol. That is, the pouring of sol into the particulate volume may occur at a greater time interval before gelation than the time interval allowed between the pouring of the particulate into a sol and the onset of gelation.
The composites can be formed as monoliths, powders or films (by preparation of the aerogel on a substrate, possibly followed by removal of the film from the substrate if a free-standing, rather than supported, film is desired). The surface characteristics of the substrate may be modified, if desired, by conventional means such as etching (e.g., chemical, mechanical, ion, or plasma) or use of a molecular primer to control the degree of adhesion between the substrate and the aerogel film.
Of particular interest in the present invention are those composite aerogels in which the average size of the particulates (as determined by electron microscopy) is smaller than the average (median) pore size (as determined by porosimetry). In such composites, it would normally be expected that the small particles could be extracted or washed out from the pores of the aerogel. However, in the present invention, those particles cannot be removed by extraction or washing.
The particulates in the aerogel composites of the present invention may also be modified to include various functional groups on the surfaces of the included particulates. For example, the surfaces of the particulates added to the gel may, before or after gelation, be functionalized by covalent bonding, chemisorption, precipitation, self-assembly, physisorption, metal-ligand coordination bonding, hydrogen bonding or electrostatic bonding to a chemical modifier. The chemical modifier may be, for example, an organic molecule, a biomolecule (e.g., a receptor site), a metal complex, a metal or ceramic precipitate, etc. This functionality can provide the composite with properties advantageous for specific uses such as metal removal, molecular recognition, biological purification, catalysis, electronics, electrical power, optical-switching, or energy transduction (e.g., photovoltaics). Generally, the modifiers used to provide conventional surfaces with those properties are well known and may be readily applied to the surfaces of the particulate guests in the present invention by those skilled in the art without undue experimentation.
Similarly, after the composite has formed, the frame of the aerogel may be surface modified, by conventional surface modification methods, such as those referenced above, to provide desired surface properties. For example, the surface of the frame may be silanized to change hydrophilicity or hydrophobicity of the frame.
The present invention may use any sol that gels to form a three-dimensional or fractal network. While most commonly used sols are in aqueous or alcoholic media and are based on metal oxides (including double metal oxides) made from metal alkoxide (including double metal alkoxide) precursors, the present invention is also useful with sols in non-aqueous or water-free media (made, for example, from carboxylate precursors by a non-hydrolytic route as described in the prior art) or non-oxidic sols (made, for example, by non-hydrolytic routes where the chalcogenide moiety is not oxygen).
Composite aerogels are platforms that provide opportunities to engineer a broad range of nanoscopic materials with specific pre-selected properties. The gel preparation scheme described herein offers multiple means to further tailor the optical, chemical, and physical properties of the guest solid, the composite wet gel or the dried aerogel by way of solution- or gas-phase modification. Additional tailoring of the composite gel architecture can be achieved by modifying the surface of the particulate guest prior to gelation. Active sites that are introduced to the surface of the guest particles prior to gelation remain accessible to external reagents after supercritical drying. Carbon-supported metal colloids in carbon-silica composite aerogels (produced by combining colloidal metal-modified Vulcan carbon with silica sol) remain accessible to CO and MeOH, and have been electronically addressed within the aerogel to catalyze redox reactions.
High-surface-area carbon blacks are typically used in fuel cells to disperse the nanoscale electrocatalyst. They may be fabricated into a fuel-cell electrode of the required geometry by combining them with a porous binder, such as poly(tetrafluoroethylene) (see, for example, M. P. Hogarth and G. A. Hards, Platinum Metals Rev. 1996, 40, 150 and K. Kordesch and G. Simader, Fuel Cells and their Applications, VCH, Weinheim 1996 incorporated herein by reference). Composite aerogels should improve existing electrocatalytic technologies because their integrated structure offers multifunctionality by providing superior access of fuel and oxidant to the dispersed, carbon-supported catalyst by way of the continuous mesoporous network, while also maintaining electronic conductivity throughout the composite.
Modified carbon-silica composite aerogels may be useful as black optical materials as well. Neither ambient nor Hexe2x80x94Ne laser light is transmitted through a 1-cm monolithic carbon-silica composite aerogel, despite its high porosity (see, for example, C. A. Morris, M. L. Anderson, R. M. Stroud, C. I. Merzbacher, D. R. Rolison, Science 1999, 284, 622, incorporated herein by reference). In contrast, native silica exhibits clarity that transmits light with little scattering. Opaque or low-reflectivity coatings and monoliths may be prepared and the wavelengths that are absorbed can then be extended beyond the visible by adsorbing molecular modifiers to the carbon. The surface of the guest particulate may also be tailored following gelation by adding solution-phase reagents to the pore-fluid washes that are performed prior to supercritical-fluid drying. For example, the surface of gold colloids larger than xcx9c20 nm remains accessible to external reagents by way of the three-dimensional mesoporous network of the composite gel. (see, for example, M. L. Anderson, C. A. Morris, R. M. Stroud, C. I. Merzbacher, D. R. Rolison, Langmuir 1999, 15, 674, incorporated herein by reference.) The base-conjugate form of the pH-sensitive dye methyl orange preferentially adsorbs (from acetone solution) to the metal surface in colloidal Au-silica composite wet gels, and not to the surface of the silica domains. The UV-visible absorption spectrum of a methyl-orange-modified colloidal Au-silica gel exhibits resolved peaks for colloidal Au and methyl orange. A more complex modification of the metal surface architecture using solution-phase reagents can be conceived that customizes these composites with molecular recognition centers for analyte specificity or tailors the colloidal metal-modified carbon-silica composites for more efficient electrocatalysis.
Modification of the composite aerogel following supercritical drying may also be employed. For composite aerogels that do not contain organic moieties, partial densification at elevated temperatures can be used to strengthen the silica network. (see, for example, E. Hummer, X. Lu, Th. Rettelbach, J. Fricke, J. Non-Cryst. Solids 1992, 145, 211; C. Lorenz, A. Emmerling, J. Fricke, T. Schmidt, M. Hilgendorff, L. Spanhel, G. Mxc3xcller, J. Non-Cryst. Solids 1998, 238, 1; A. Martino, S. A. Yamanka, J. S. Kawola, D. A. Loy, Chem. Mater. 1997, 9, 423; M. T. Reetz, M. Dugal, Catal. Lett. 1999, 58, 207, T. Woignier, J. Phalippou, and M. Prassas, J. Mater. Sci., 1990, 25, 3118, and J. Cross, R. Goswin, R.. Gerlach, J. Fricke, Rev. Phys. Appl. 1989, 24, C4-185, incorporated herein by reference). Silica or colloidal Au-silica composite aerogels heated to 900xc2x0 C. shrink (xcx9c50 % reduction in the size of the monolith), but the primary loss in free volume, as determined by N2-physisorption measurements, occurs by collapse of the micropores (pores less than 2 nm), while most of the mesoporosity (2- to 50-nm pores) is preserved. Preserving the mesoporous free volume means that the most facile mass-transport pathways through the composite aerogel for gas- or solution-phase reactants remain unaltered. Furthermore, the composite constitutes a rigid solid architecture, such that the silica aerogel structure and metal particle size distribution are retained in partially densified colloidal Au-silica composite aerogels.(see, for example, M. L. Anderson, D. R. Rolison, C. I. Merzbacher, SPIE Engineered Nanostructural Films and Materials 1999, 3709, 38, incorporated herein by reference).
Partially densified composite aerogels are sufficiently durable that they remain intact upon reimmersion into liquids. This durability can be demonstrated by preferentially adsorbing methyl orange from solution onto the Au surface in partially densified colloidal Au-silica composite aerogels, analogously to the specific adsorption of the dye in colloidal Au-silica composite wet gels, as described above. This surface-specific modification is consistent with the retention of a continuous mesoporous network in silica-based composite aerogels, even after partial densification, as indicated by N2-physisorption studies comparing as-prepared and partially densified aerogels. On the basis of these independent measurements of the total sample pore volume that is contributed by micro- and mesopores, nearly 60% of the 500xc2x0 C.-annealed aerogel mesoporosity is preserved in the 900 xc2x0 C.-partially densified aerogel, while  less than 15% of the microporous volume is retained in the partially densified sample.
The feasibility of optical or calorimetric sensing with composite gels has been verified by using a combination of modification steps. A multistep modification strategy has been demonstrated by thermally densifying 50-nm colloidal gold-silica composite aerogels and modifying the colloidal Au guests with methyl orange by immersion of the partially densified composite aerogel into a nonaqueous solution of the dye. Analogously to the wet composite gels discussed above, resolved peaks for the Au plasmon resonance and the methyl orange (base-conjugate form) absorbance are seen in the UV-visible spectrum of a methyl-orange-modified colloidal gold-silica composite aerogel that was thoroughly rinsed with acetone, then air-dried. Exposing the dye-modified, air-dried composite to HCl vapor produces a red-shift in the dye""s absorption, corresponding to its protonation. The gas-phase acid molecules may be detected either visually or by instrumental colorimetry. Visual detection is possible because although the surface coverage of the adsorbed dye is quite low (typically  less than 0.1 of a monolayer), the surface-to-volume ratio of the composite is enormous, which brings the effective concentration of the dye in the modified composite aerogel to millimolar levels. Color changes are rapid, because of the high porosity, and are readily discerned visually. Upon uptake of methyl orange, the color of the colloidal Au-silica composite aerogel changes from cranberry to peach (again, no methyl orange is retained in partially densified pure silica), and a further color change from peach to bright pink occurs within seconds of exposure of the dye-modified Au-silica composite aerogel to HCl vapor.